The viola is a versatile instrument which can serve as a "3rd violin", bridge the voicing between the violin and cello, or double the cello line or second violin for greater reinforcement.
The tone quality of the viola is slightly darker and has almost a muted quality to it compared to the violin. Its lower range will match the cello and its upper range will blend well with the violin.
The viola plays primarily in alto clef, higher passages can be notated in treble clef. For ensembles, I recommend avoiding writing in above the practical ensemble range. If you need to write higher, revoice your arrangement and use the second violins.